Digital subscriber line (DSL) technology, often referred to as “broadband”, is a family of services that provides high speed digital data transmission over the metallic twisted copper pairs that form part of a local telephone network. DSL is commonly used to provide a customer's home with a network connection, typically to the Internet via an Internet Service Provider (ISP).
Some DSL lines, especially longer rural lines, are susceptible to interference resulting in instability in the connection. A DSL line can lose synchronisation when the amount of noise on the line is such that the data (signal) being transmitted cannot be processed correctly. Line management techniques can help address this problem and stabilise a line at the expense of a lower synchronisation rate. One such technique is known as Dynamic Line Management DLM.
DLM measures the line parameters, such as the error rates and train/resynchronisation events, and adjusts line configuration parameters accordingly to try and stabilise the line, which has the effect of reducing the synchronisation rate. DLM applies one of a limited number of profiles to the line to improve the stability. One method of achieving this is by setting a target signal to noise ratio margin, or target margin, which affects the maximum rate the line can synchronise at. A high target margin will result in a more stable line, and a low target margin a less stable line (all other factors being equal). The effect is more pronounced on problematic or longer lines.
In some typical forms of DLM, the profiles applied are limited in the sense that only a small number of relatively coarse target margins are used. An example of the range of target margins typically used might be 3 dB, 6 dB, 9 dB, 12 dB and 15 dB. The profile applied by DLM is used when a line resynchronises. Line resynchronisations can occur when the line can no longer maintain synchronisation, for example as result of excessive noise, interference, or a fault on the line.
The signal to noise ratio (SNR) of the line affects the synchronisation rate, as the bit allocation methods used when a line resynchronises are dependent on the SNR (of each frequency bin) when the line starts up.